I'm a Mac newbie (after years of IBM PC) and have never worried before about keeping people out of my computer, but now it's important. Operating system is OS X 10.5.8. When a friend helped me transfer my files to this computer, we set up an admin password, which I of course know. Also, it knows me as a user by my name. I want to be able to shut down the computer, and then when I power it back on, for it to ask for a password and only let me in, nobody else. Believe it or not, I can't figure out how to do this. Please be kindergarten simple.